docs: def variance(self, param):
          """
          Return the variance in a given parameter as found by the fit.
  
          :param param: ``Parameter`` Instance.
          :return: Variance of ``param``.
          """
          param_number = self.model.params.index(param)
          try:
              return self.covariance_matrix[param_number, param_number]
          except TypeError:
              # covariance_matrix can be None
              return None